In electronic fuel injection control methods in which the operating state of an internal combustion engine is monitored and a fuel injection amount is controlled by way of an electrical circuit, several types of injection characteristics are used, including for example starting injection at the start of the engine, injection in the ordinary manner, asynchronous injection, and stop of the injection. Each of these injection characteristics is carried out under respective conditions specified by signals from the sensors monitoring the operating state of the engine, and the fuel injection amount is controlled on the basis of one of the injection characteristics in accord with existing conditions as evidenced by the sensor signals.
In such conventional control methods, the controls which typically invoke starting injection at the start of the engine and ordinary injection during ordinary operation have been disclosed in the Official Gazettes of Japanese Patent Unexamined Publication Nos. 61531/1975 and 63327/1984. Namely, for example, as shown in FIG. 1, upon starting the engine, a starter switch is turned on and invokes the starting fuel injection characteristic to facilitate starting of the engine, thereby increasing a fuel injection amount and improving a starting performance. After completion of the starting of the engine, the starter switch is turned off and a fuel injection in the ordinary manner is performed when the rotating speed N.sub.e of the engine is equal to or greater than a predetermined rotating speed N.sub.s. However, starting injection is always carried out and the fuel injection amount is increased until the engine reaches the set rotating speed N.sub.s in order to promptly stabilize the engine when the rotating speed N.sub.e of the engine is lower than the predetermined rotating speed N.sub.s, even after completion of the starting of the engine.
When the engine is being operated by injecting the ordinary amount of fuel after starting of the engine, it is possible that the engine rotating speed N.sub.e may temporarily decrease and become lower than the predetermined rotating speed N.sub.s, for example due to the application of a load, such as engagement of a clutch in order to initiate movement of a vehicle or the like. Thus, in the foregoing conventional control methods, a low engine speed always causes the injection characteristic to be changed from the characteristic for fuel injection in the ordinary manner to the characteristic for fuel injection at the start of the engine, thereby causing the amount of fuel injected to be increased. Consequently, there is the drawback that a large amount of a rich air-fuel mixture is supplied to the engine and the engine vibrates due to the abnormal burning, causing operating efficiency to be lost.